Page 384 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

348 part III The Earth–Atmosphere Interface
The Earth–atmosphere interface is the meeting 18th century and later amplified by Charles Lyell in his
place of internal and external processes that build
up and wear away landscapes. As stated in the Part III introduction, the endogenic system consists of processes operating in Earth’s interior, driven by heat and radioactive decay, whereas the exogenic system consists of processes operating at Earth’s surface, driven by solar energy and the movement of air, water, and ice. Geology is the science that studies all aspects of Earth—its history, composition and internal structure, surface features, and the processes acting on them. Our overview of Earth’s en- dogenic, or internal, system in this chapter covers geol- ogy essentials, including the types of rocks on Earth and their formation processes and the theory of plate tecton- ics. These essentials provide a conceptual framework for the spatial study of the lithosphere in physical geography.
The study of Earth’s surface landforms, specifically their origin, evolution, form, and spatial distribution, is geomorphology—a subfield of both physical geography and geology. Although geomorphology is primarily re- lated to the exogenic, or external, system, our study of Earth’s exterior begins with an explanation of Earth’s interior, the basic materials and processes that shape Earth’s surface.
In this chapter: Earth’s interior is organized as a core surrounded by roughly concentric shells of material. It is unevenly heated by the radioactive decay of unstable elements. A rock cycle produces three classes of rocks through igneous, sedimentary, and metamorphic pro- cesses. A tectonic cycle moves vast sections of Earth’s crust, called plates, accompanied by the spreading of the ocean floor. Collisions of these plates produce irregular surface fractures and mountain ranges both on land and on the ocean floor. This movement of crustal material results from endogenic forces within Earth; the surface expressions of these forces include earthquakes and vol- canic events.
The Pace of Change
In Chapter 11, we discussed paleoclimatic techniques that establish chronologies of past environments, en- abling scientists to reconstruct the age and character of past climates. An assumption of these reconstructions is that the movements, systems, and cycles that occur today also operated in the past. This guiding principle of Earth science, called uniformitarianism, presupposes that the same physical processes now active in the envi- ronment were operating throughout Earth’s history. The phrase “the present is the key to the past” describes the principle. For example, the processes by which streams carve valleys at present are assumed to be the same as those that carved valleys 500 million years ago. Evidence from the geologic record, preserved in layers of rock that formed over millennia, supports this concept, which was first hypothesized by geologist James Hutton in the
seminal book Principles of Geology (1830).
Although the principle of uniformitarianism applies
mainly to the gradual processes of geologic change, it also includes sudden, catastrophic events such as mas- sive landslides, earthquakes, volcanic episodes, and as- teroid impacts. These events have geological importance and may occur as small interruptions in the generally uniform processes that shape the slowly evolving land- scape. Thus, uniformitarianism means that the natural laws that govern geologic processes have not changed throughout geologic time even though the rate at which these processes operate is variable.
The full scope of Earth’s history can be represented in a summary timeline known as the geologic time scale (Figure 12.1). The scale breaks the past 4.6 billion years down into eons, the largest time span (although some refer to the Precambrian as a supereon), and then into increasingly shorter time spans of eras, periods, and epochs. Major events in Earth’s history determine the boundaries between these intervals, which are not equal in length. Examples are the six major extinctions of life forms in Earth history, labelled in Figure 12.1. The timing of these events ranges from 440 million years ago (m.y.a.) to the ongoing present-day extinction episode caused by modern civilization (discussed in Chapter 19; for more on the geologic timescale, see www.ucmp.berkeley.edu/exhibit/ geology.html).
Geologists assign ages to events or specific rocks, structures, or landscapes using this time scale, based on either relative time (what happened in what order) or numerical time (the actual number of years before the present). Relative age refers to the age of one feature with respect to another within a sequence of events and is deduced from the relative positions of rock strata above or below each other. Numerical age (some- times called absolute age) is today determined most often using isotopic dating techniques (introduced in Chapter 11).
Determinations of relative age are based on the gen- eral principle of superposition, which states that rock and unconsolidated particles are arranged with the youngest layers “superposed” toward the top of a rock formation and the oldest at the base. This principle holds true as long as the materials have remained undisturbed. The horizontally arranged rock layers of the Grand Can- yon and many other canyons of the U.S. Southwest are an example. The scientific study of these sequences is stratigraphy. Important time clues—for example, fossils, the remains of ancient plants and animals—lie embed- ded within these strata. Since approximately 4.0 billion years ago, life has left its evolving imprint in the rocks.
Numerical age is determined by scientific meth- ods such as radiometric dating, which uses the rate of decay for different unstable isotopes to provide a steady time clock to pinpoint the ages of Earth ma- terials. Precise knowledge of radioactive decay rates